In conventional image forming apparatus in which a latent image is written on an image carrier, it is known to employ an LED (light emitting diode) array as writing means. In case of employing light emitting elements such as an LED, it is necessary to pay attention to the relation between the luminance (amount of light) and the life duration of each light emitting element. That is, the life duration can be increased by reducing the luminance of the light emitting element. In this case, however, there is a problem that the amount of light for exposure is insufficient to form image. When the luminance of the light emitting element is increased, enough amount of light for exposure for forming image is obtained. In this case, however, there is a problem that the life duration is shortened.
For this, the development of material for obtaining light emitting elements capable of providing large luminance and having long life duration has been encouraged. However, in the present state of affairs, it is too expensive to achieve the practical use. In this connection, a line head (optical head) of multiple exposure type in which each pixel is exposed by a plurality of light emitting elements has been developed. As an example (1) of such line heads of multiple exposure type, Japanese Patent Unexamined Publication No. S61-182966 discloses a recording array head on which light recording elements are aligned in a plurality of rows in the rotational direction of a photoreceptor drum. An image data is overlappingly recorded at the same pixel by shifting the light emitting recording elements in the direction of the rows with moving the photoreceptor drum. The example (1) has an advantage that higher speed image formation is achieved even using light recording elements with low light-emitting output.
Another example (2) is disclosed in Japanese Patent Unexamined Publication No. S64-26468 in which an EL element panel is composed of EL element group of 20 dots×640 dots (vertical×horizontal). The EL element group is driven at a speed same as the moving speed of a photoreceptor for every line. Accordingly, the amount of light irradiated on a single pixel is twentyfold the amount of light emitted from each EL element. This example also can cope with high speed of image formation because the amount of light of exposure per pixel is increased.
Another example (3) is disclosed in Japanese Patent Unexamined Publication No. H11-129541 and is a print head on which LEDs are aligned in a plurality of lines such that multiple exposure is made on each pixel by moving the print head in the main scanning direction. In this example, since the multiple exposure is conducted, variations in amount of light among the respective LEDs can be equalized, thereby improving the image quality. Still another example (4) is disclosed in Japanese Patent Unexamined Publication No. 2001-260411 and is an optical printer head on which plural lines of LED array chips are aligned. The gradation of each pixel can be changed among three levels by turning ON or OFF the LED array chips on each line.
The aforementioned examples (1) and (2) relate to a technology of monochrome image formation and therefore have a problem that gradation control for neutral density is impossible. The example (3) relates to a technology of a serial type in which the line head is driven and therefore has a problem of having a complex driving mechanism. The example (4) relates to a technology in which the LED array chips on each line are turned ON and OFF and therefore has a problem of complexity of the control circuit.
Since the number of light emitting elements in the line head of multiple exposure type are greater than that of a line head of normal exposure type and it is necessary to control the light emitting elements synchronously with the movement of the photoreceptor, there is a problem of complexity of the control circuit for conducting the data processing and it is therefore difficult to achieve higher speed light emitting control. Especially, in case that a line head (optical head) of multi-exposure type is employed for color image formation, the amount of data to be processed must be several fold that in the case of ON-OFF control because the gradation control for each pixel is sometimes required. This makes the speeding up of emission control further difficult. In case of line head of multiple exposure type, it is required to send a large amount of data processed by a data processing unit to the line head, thus increasing the number of wires between the line head and the body of an image forming apparatus. Accordingly, it is necessary to employ an interface capable of supporting the high-speed processing, thus increasing the cost.
The present invention was made in view of the aforementioned problems of conventional techniques and the object of the present invention is to provide an image forming apparatus and an image forming method which are directed to simplify the circuit structure and to speed up the light emitting control during the exposure of pixels on an image carrier in multiple exposure manner capable of outputting gradation.